The present invention relates to a method for connecting a plurality of battery cells of a battery designed as an electrochemical energy store. The invention also relates to a battery system with a battery designed as an electrochemical store with a plurality of battery cell units which in each case comprise a battery cell and a battery cell monitoring module allocated to the battery cell.
FIG. 1 shows a battery system 10 known from the prior art which comprises a battery 11 with a plurality of battery cell units (Smart Cell Unit SCU) 20 which in each case have a battery cell 21 and a battery cell monitoring module (battery cell electronic module or battery cell electronics) 22 allocated to the battery cell 21. In order to simplify the illustration from FIG. 1, only two battery cells have been drawn and denoted in each case with the reference number 20. The battery cell monitoring modules 22 enable an individual control of the individual battery cells 21. In order to generate an output voltage (total output voltage) U of the battery 11, which also serves as the output voltage U of the battery system 10, the battery cell monitoring modules 22 are interconnected in a series circuit via a connection path. The battery system 10 furthermore comprises a Central Control Unit (CCU) 30 to control the battery system 10.
In order to generate a controlled output voltage (total output voltage) U of the battery 11, individual battery cells 21 are activated in each case by means of the allocated battery cell monitoring module 22, i.e. the battery cells 21 can be incorporated into the series circuit in each case in positive or negative polarity relative to the tap of the output voltage U of the battery 11. In order to generate a controlled output voltage (total output voltage) U of the battery 11, individual battery cells 21 are furthermore deactivated in each case by means of the allocated battery cell monitoring module 22, i.e. the battery cells 21 to be deactivated are disconnected from the series circuit by electrically connecting the connection terminals of each battery cell 21 that is to be deactivated by means of the allocated battery cell monitoring module 22, whereby the corresponding battery cells 21 are bypassed. The battery cells 21 connected to the series circuit can consequently in each case be in a switching state referred to as “positively connected” or in a further switching state referred to as “negatively connected”. Furthermore, the battery cells 21 disconnected from the series circuit may be in a switching state referred to as “bypassed”.
In battery systems 10 (SmartCell battery systems) of this type, the decision regarding the change of the switching state of the battery cells 21 is taken locally in the respective battery cell monitoring modules 22. The actual control function is implemented by the central control unit 30 which is designed as a central controller implemented at low cost.
In the battery system 10, a first control parameter P1 and a second control parameter P2 are specified via a communication path 31 designed as a one-way communication interface via which only a single message comprising the existing control parameters P1 and P2 is transmitted from the central control unit 30 to all battery cell monitoring modules 22. All battery cell monitoring modules 22 receive the same message and either autonomously connect the respectively allocated battery cells 21 to the series circuit or bypass the respectively allocated battery cells 21 by means of the corresponding switches (not shown) present in each case in the battery cell monitoring modules 22. According to a control algorithm, the central control unit 30 specifies the two control parameters P1, P2 in the form of two numerical values between 0 and 1 which are transmitted via the communication path 31 from the central control unit (CCU) 30 to the battery cell monitoring modules (SCU) 22 and are likewise received by all battery cell monitoring modules 22. Here, 0≤P1≤1 and 0≤P2≤1 apply.
In each battery cell monitoring module 22, an equally distributed random process is carried out which interprets P1 as a first probability referred to as the activation probability with which each deactivated battery cell 21 will be activated, and P2 is interpreted as a second probability referred to as the deactivation probability with which each activated battery cell will be deactivated. The central control unit 30 tracks the control parameters P1 and P2 so that the smallest possible difference (control difference) occurs between an existing output voltage U of the battery 11 and a desired output voltage Us of the battery 11.
In addition to the generation of a controlled output voltage U of the battery 11, a simple extension of the control algorithm executed by the central control unit 30 can be performed in such a way that an active battery cell functional state balancing (battery cell balancing) is achieved through the simultaneous use of a weighted usage duration for the battery cells 21.
To do this, each battery cell monitoring module 22 scales the relevant control parameter P1 or P2, i.e. the identically received control parameter P1 or P2 selected depending on the switching state of the allocated battery cell 21, depending on a quality factor which is calculated depending on a state of charge (SOC) and a state of health (SOH) of the allocated battery cell 21. As a result, deactivated battery cells 21 with a higher quality factor are activated during a discharging process with a higher probability than battery cells 21 with a lower (lesser) quality factor. Conversely, battery cells 21 with a lower quality factor are deactivated during a discharging process with a higher probability than battery cells 21 with a higher quality factor. On average over time, battery cells 21 with a lower quality factor are less frequently drained, as a result of which an active battery cell functional state balancing of the battery cells 21 is achieved.
In the implementation of the battery cell functional state balancing method described above, it has emerged that a battery cell functional state balancing can be achieved only if the quality factors of the battery cells 21 differ significantly from one another. If a battery cell functional state balancing method depending on the states of charge of the battery cells 21 is used, the effect of a battery cell functional state balancing carried out in this way with the occurrence of state of charge differences between the battery cells 21 which are less than 5% is then barely recognizable. Through the use of a statistical control algorithm (regulation algorithm) as described above, the battery cells 21 are drained depending on the statistical fluctuation. Investigations to date have shown that this effect is predominant in the hitherto used battery cell state balancing method. As a result, the states of charge of the battery cells 21 of a battery 11 of a battery system 10 known from the prior art always differ from one another in the range from 0 to 5%.